The goal of this proposal is to identify underlying molecular and cellular mechanisms of how aging and menopausal hormone treatment (MHT) affect platelet functions in newly menopausal women and how these changes relate to progression of sub-clinical disease. In animal studies conducted by our group and others, estrogen reduced activation and secretion of vasoconstrictor and mitogenic cytokines from platelets and reduced myointimal hyperplasia in response to arterial endothelial injury. However, similar mechanisms of how estrogen affects platelet activity and their relationship to vascular anomalies have not been validated in humans. Proposed experiments fill several important gaps in knowledge identified in the 2007 Position Statement of the North American Menopause Society by providing the first mechanistic, longitudinal analysis during menopausal aging of ways that MHT affects platelet activity in women in whom arterial anatomy is measured. The central hypothesis is that platelet activity is regulated in part by estrogen status (replete or deplete) and that increases in platelet procoagulant activity contribute to increased formation of lesions in the arterial wall. This hypothesis is based on the findings that thrombotic risk increases with age and that cardiovascular disease increases exponentially in women in the estrogen-deplete condition of menopause. Proposed experiments exploit a unique opportunity to evaluate healthy, newly menopausal women (ages 42- 58 years) enrolled in the Kronos Early Estrogen Prevention Study (KEEPS), a primary prevention study to determine whether treatment with either oral conjugated equine estrogen or transdermal 172-estradiol reduces carotid intimal medial thickness and coronary arterial calcification. This project is novel in several aspects: 1) it will provide the first mechanistic, longitudinal study of menopausal aging and platelet procoagulant activity; 2) it will compare effects of oral vs. transdermal estrogen preparations on platelet procoagulant activity; 3) it will define genetic factors that determine individual responses to estrogen using a polygenomic approach; finally, 4) it will relate changes in platelet procoagulant activity to vascular anomalies, thus allowing our observations in experimental animals to be translated to humans. These approaches will provide new mechanistic insights into how estrogen affects progression of cardiovascular disease and have the potential to lead to development of novel diagnostic strategies to establish a benefit/risk profile for an individual woman contemplating use of MHT.This project will examine how aging and hormones used to treat menopausal symptoms change the ability of the blood to clot in women enrolled in the Kronos Early Estrogen Prevention Study (KEEPS). This project is unique because it focuses on cellular parts of the blood called platelets, whereas previous hormone studies only examined proteins in the blood that cause clot formation. This study addresses three important needs for research identified by the North American Menopause Society: 1) it examines estrogen effects on hypercoagulability of the blood, specifically platelets; 2) it compares different formulations of estrogen, and 3) it uses a polygenomic approach to identify persons at risk for thrombosis induced by estrogen. Thus, results will identify which women could benefit most with the least harm from hormone use in early menopause.